Looking only a decade ahead, we know that there will be more elderly and more children in secondary schools. These children have already been born, and we know where they are because they registered with local doctors. There will be more elderly people, partly because 70-80 years ago the birth rate was also rising and partly because the expectation of life has continued to increase despite the dampening effects of obesity and smoking. A rise in numbers of residents aged over 65 and of the very elderly, aged over 85, is expected in almost every part of the UK.

The biggest unknown when it comes to the demographic future is migration. Will families with babies continue to move from the local flats as their children become mobile? Will the high income families that have established gentrified inner-city enclaves remain there as they age? Will the next group of immigrants swept to Britain by war or economic competition settle in the same urban places?

The trick for local planners is to distinguish how much of future demographic change is predetermined, how much is effectively uncertain and unpredictable, and how much can be seriously influenced – and then to plan accordingly. The mantra should be: predict what is not under your control, plan what is under your control.

Official demographic projections for local authorities are traditionally trend-based; they continue into the future the patterns of recent demographic change. They are said to be free of assumptions about policy, but on the contrary they assume that any impact which policy has had on the recent past will continue into the future.

When it comes to land, the dictum 'plan what you can control' has long been at the forefront of planning policy. It is standard practice to plan the future number of housing units and where they will be built – and to calculate the impact this will have on population change – attempting to involve developers in the impacts on education and transport.

The same approach could be used more often for locating new homes for older people, and for education which these days influences a significant number of family housing choices. The tools for this job are already available.

The impact of social policy on population trends reflects not only on the number of people but on households,the main unit of consumption and expenditure. The time that youngsters stay with parents before setting up their own home has been lengthening. The feasibility of paying for a care home without spending one's entire wealth has diminished, adding pressure to care in the home by relatives or others.

These tensions have been exacerbated by cuts to local research (you can share your own experiences of this here) while localism has thrown inexperienced district councils at the feet of developers' lobbying presentations. Who is actively promoting quality in local pubic planning and demographic forecasting?

Ludi Simpson is professor of population studies at the University of Manchester. He gave evidence to the House of Lords committee inquiry into how demographic change will affert public services.